Austenitic stainless steels adapted for exhaust valve applications

ABSTRACT

Austenitic stainless steels containing chromium, nickel, carbon and phosphorus in controlled amounts offer a combination of corrosion resistance, hardness, strength, fabricability, etc., which render them particularly suitable as exhaust valve steels.

United States Patent Larson, Jr. et al.

[ Mar. 14, 1972 AUSTENITIC STAINLESS STEELS ADAPTED FOR EXHAUST VALVE APPLICATIONS Floyd G. Larson, Jr., Ringwood, N.J.; John J. de Barbadillo, 11, Suffern, NY.

Inventors:

Assignee: The International Nickel Company, Inc.,

New York, N.Y.

Filed: Mar. 25, 1970 Appl. No.: 22,631

US. Cl. ..75/128 P, 75/128 R, 148/37 Int. Cl ..C22c 39/20, C22c 39/ 14 Field of Search ..75/128 P, 128 R; 148/37 fi wcsmr l/OSPA'OQUS [56] References Cited UNITED STATES PATENTS 2,799,577 7/1957 Schempp ..75/128 P 2,437,478 4/1969 Moskowitz ..75/128 P Primary Examiner-Hylsnd Bizot Attomey-Maurice L. Pinel [57] ABSTRACT Austenitic stainless steels containing chromium, nickel, carbon and phosphorus in controlled amounts offer a combination of corrosion resistance, hardness, strength, fabricability, etc., which render them particularly suitable as exhaust valve steels.

15 Claims, 1 Drawing Figure AUSTENITIC STAINLESS STEELS ADAP'IED FOR EXHAUST VALVE APPLICATIONS As those skilled in the art are aware, for a not inconsiderable number of years much importance has attached to the development of new and improved valve steels, notably for exhaust (as distinct from inlet) valves in respect of high-performance engines. And if, as seems possible, more severe operating conditions are encountered in the future, particularly those imposed by higher combustion temperatures, there will continue to be a need for steels capable of delivering new dimensions of performance with regard to such characteristics as corrosion behavior, high-temperature hardness and strength, etc., characteristics indicative of long valve life.

Steels used in the production of exhaust valves should afford a diverse combination of metallurgical properties. If there is a primary prerequisite for such materials, whether for duplex (the valve type to which the present invention is primarily directed) or single-piece valves, it is the ability to resist the destructive influence of the combustion products of leaded fuels at temperatures on the order of, say, 1,350 to l,500 F. And as the temperature of operation is increased, so too is the aggressiveness of this particular corrosive environment.

As to other properties, high hardness at elevated temperatures, often termed hot hardness, is particularly essential; otherwise, excessive indentation may occur at the seating surface which, in turn, can bring about premature failure. In this regard, a Rockwell hardness of at least R,, 45 is desirable at temperatures on the order of l,400 F. It might be added that although at ambient temperatures hardness is not as significant for duplex valves as for the single-piece type, nonetheless, levels below about R 2830 are considered inadequate even for the duplex versions at ambient temperatures. Hardness is often used as an indicia in respect of other properties and at lower hardness levels it would not be unexpected to find such other characteristics wanting.

With respect to strength properties, steels of the type under consideration again must particularly manifest good strength at the high temperature levels above noted. Resistance to stress rupture is the criterion quite commonly used for evaluation purposes and to be acceptable in accordance herewith a steel should exhibit a stress rupture life of at least 50 hours, preferably at least 75 hours, under a stress of 35,000 psi. at 1,350 F. and also under a stress of 10,000 psi. at l,500*F.

In addition to all the foregoing, it is noteworthy of mention that since valve steels are most generally used in the wrought form good hot workability, including resistance to hot shortness, and ease of fabricability are virtually indispensible. Too, unless such a steel is weldable it would be completely inadequate for duplex exhaust valve production.

In approaching the overall problem and in view of the corrosion characteristics and mechanical properties of conventional exhaust valve steels (see, for example, Metals Handbook, Vol. 1, 8th ed., pages 626-634) consideration was given to certain cobalt-base and other austenitic alloys in search for an improved combination of properties. But due to cost factors this approach was not pursued. An economical alloy being a prime target, this emphasized the necessity of developing a steel with the above discussed properties by low cost processing, for example, the use of standard air melting procedures rather than finding recourse in the more expensive vacuum techniques.

In any event, it has now been discovered that corrosion resistant, strong, hot workable and weldable austenitic stainless steels of novel composition can be produced for exhaust valve applications provided the steels contain correlated percentages of chromium, nickel, carbon, phosphorus, etc., as set forth hereinafter.

it is an object of the present invention to provide new and improved stainless steels, particularly stainless steels meeting the objectives above described.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which FIG. 1 is a graphical delineation of various steel compositions within the invention as opposed to those beyond the scope thereof.

Generally speaking, and in accordance with the present invention, austenitic stainless steels contemplated herein contain from 18 to 30 percent chromium, about 20 to 40 percent nickel, from 0.25 percent and up to less than about 0.95 percent carbon, from 0.1 to about 0.35 percent phosphorus, with the proviso that the carbon and phosphorus are correlated so as to be represented by a point within the area ABDEFA of the accompanying drawing, up to about 0.5 percent silicon, up to 2 percent manganese, and the balance essentially iron.

In carrying the invention into practice should the chromium content be below about 18 percent, resistance to the combustion products of leaded fuels is inferior whereas if the chromium percentage should significantly exceed 30 percent then there is a risk that detrimental amounts of either or both delta ferrite and sigma or alpha chromium will be found in the microstructure of the alloys, microstructural phases which detract from tensile ductility and toughness, particularly at high temperatures. A chromium range of from about 25 to 29 percent gives excellent results.

Below about 20 percent nickel, resistance to the aggressive effects of lead oxide corrosion unnecessarily suffers and there is a greater tendency for unwanted delta ferrite to form. For continuously achieving good resistance to lead corrosion attack the chromium and nickel should be correlated such that the sum of 2.8 times the percentage of chromium (2.8% Cr) plus the percentage of nickel Ni) is not less than about 92. The benefits derived from nickel levels above about 40 percent are not sufficiently attractive from an economic viewpoint to warrant using higher percentages. A particularly useful and economical nickel range is from about 25 to 33 percent.

Of considerable importance is the effect of carbon and phosphorus. It has been determined that carbon promotes hardness, strength and also resistance to lead oxide attack; however, in the absence of phosphorus, it does not in a satisfactory manner confer sufficient high temperature stressrupture strength. To achieve this desired characteristic an otherwise excessively high level of carbon, e.g., above 1% or more, would be necessary, a level causative of or which promotes poor fabricability characteristics, thus rendering production of wrought valves and other articles difficult at best. However, though our present understanding of the metallurgical phenomenon involved may not be complete, it is thought that phosphorus contributes to a synergistic effect or at least intensifies the carbon effect such that strength and hardness at temperatures upwards of l,350-l ,500 F. are dramatically influenced. This obviates, fortunately, the necessity of possibly using high carbon contents and consequently minimizes hot working difficulties.

But this positive role portrayed by phosphorus must be controlled, indeed counterbalanced, since it too has been found not to be without drawbacks. Phosphorus enhances stress rupture strength (as well as intensifying the carbon effect), but if present to the excess, say, over 0.35 percent, and certainly over 0.4 percent, it undesirably detracts from corrosion resistance and also promotes hot workability problems. Accordingly, the carbon should always exceed the phosphorus content by a ratio of at least 1.25:1 (as required by the accompanying drawing), nd advantageously by a ratio of at least 1.5: 1. Furthermore, while the total percentage of carbon plus phosphorus should not fall below about 0.4 percent, it is most beneficial that it not be less than 0.45 percent, the carbon content being advantageously at least 0.3 percent but not exceeding 0.5 percent or 0.6 percent with the phosphorus being at least 0.12 percent or 0.15 percent but not exceeding about 0.3 percent. Moreover, in striving for overall optimum results it is deemed that the sum total of these constituents beneficially should not exceed 0.75 percent. Given the foregoing considerations of corrosion resistance, high temperature hardness and stress rupture strength, and hot workability, the carbon and phosphorus should be correlated so as to represent a point falling within the area ABCGHFA of the accompanying drawmg.

Mention should perhaps be made that certain of the prior art literature reflects that it is virtually indispensible to limit the presence of silicon in automobile exhaust valve steels to low values owing to its detrimental effect on corrosion. Usually a nominal value of 0.1 percent maximum is prescribed although amounts up to 0.2 percent or 0.25 percent. on occasion have been set. Although this heretofore imposed requirement can, of course, be observed (and it is so recommended) in accordance herewith, it is not of an absolute necessity. A more tolerable silicon range does have the advantage that silicon can be used, if desirable, for its effects as a powerful deoxidant; also the avoidance of using scrap materials is relaxed to the extent that silicon contents above 0.1 percent and up to 0.3 percent or 0.4 percent can be toleratedv But most preferably, the sum of total phosphorus and silicon should be controlled as to not exceed about 0.65 percent; otherwise, loss of resistance to the corrosive attack of leaded fuels becomes an ever increasing risk.

With respect to other elements, it is to be understood that in referring to iron as constituting the balance or balance essentially" of the instant steels, the presence of other constituents is not excluded, such as those commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely effect the basic characteristic of steels. Hydrogen, oxygen and sulfur should, however, be maintained to low levels consistent with good steelmaking practice. Neither aluminum nor titanium is required and if present to the excess can impair corrosion resistance. It is recommended that these elements not exceed about 0.1 percent and 0.2 percent, respectively. Strong carbide formers, columbium, vanadium, tungsten and molybdenum, are unnecessary and can lower the as-aged hardness and strength as well as introduce a loss in resistance to lead oxide attack. Small amounts of such constituents as boron up to 0.01 percent and zirconium up to 0.05 percent can be present to impart their usual benefits.

As mentioned above herein, the subject steels can be prepared using standard air-melting practice although, of course, conventional vacuum procedures can be employed. Concerning hot working, temperatures much in excess of 2,250 F. or below 1,800 F. should be avoided. However, it has been found that in the initial hot working stages such as by forging, should the temperature exceed 2,150 F. to any significant degree, there is risk of experiencing severe hot shortness. This is due, it is believed, to the liquation of an austentitic-carbide-phosphide ternary eutectic. On the other hand, at temperatures below 1,800 E, the inherent deformation resistance of the alloys pro se to hot working is too great. Moreover, it has been found that cold cracking may also be a problem in the rolling of, say, bar stock as a result of having used temperatures below 1,800 F. In subsequent hot working to say, bar and other forms, temperatures up to about 2,250 P. can be employed. lngot conditioning prior to hot working is most beneficial. This can be accomplished by surface grinding to thereby remove hot tears, pores, and ingot hot top.

With regard to heat treatment, solution temperatures of at least about 2,200 F., for example, at 2,300 to 2,350 F. are satisfactory. Lower temperatures can be employed, e.g., down to about 1,800 F., but without the benefits derived from the higher solution-treating temperature. This seemingly is attributable to the fact that the higher temperatures ultimately result in greater hardness and strength upon aging, other factors remaining equal. This enables lower phosphorus contents to be used than otherwise might be the case. This in turn results in improved workability and weldability characteristics. Upon cooling from solution treatment, the steels are then age hardened over a temperature range of about 1,l to about 1,400 F. for about one hour to 48 hours, the longer aging times being used with the lower temperatures. Aging at 1,250" to 1,350 E, e.g., 1,300 F. for about 4 hours is quite satisfactory.

For the purpose of giving those skilled in the art a better understanding of the invention the following description and illustrative data are given.

Air induction melts were prepared using, for the most part, electrolytic nickel, shieldalloy chromium, spectographic grade carbon, ferrophosphorus, ferrosilicon, and a good commercial grade of iron. The melts were deoxidized with ferrosilicon and cast as l ir-inch-diameter bars in cast iron molds. Thereafter, specimens were solution treated, water quenched, and aged. All of the alloy specimens other than Alloys 1 and 2 were cast specimens solution treated for one hour at 2,100" F. and aged 16 hours at 1,200 F., Alloys 1 and 2 being in the wrought form and solution treated at 2,300 F. for about one hour and then aged at l,300 F. for about 4 hours.

Concerning the corrosion test, approximately half of a 40- gram charge of lead oxide was placed in a Magnorite crucible whereupon the test specimen was inserted and the rest of lead oxide was added. The test temperature was about l,675 F. and the crucibles were held at temperature for approximately 1 hour, thereafter being removed and allowed to cool. (Furnace temperature control was maintained approximately i 5 F.). Lead oxide and corrosion products were removed from the specimens by rough scraping followed by an electrolytic molten caustic bath descaling. Compositions and results are reported in Table I.

TABLE I Wt. loss in Percent P 1 hr., 1,675 F., Alloy Cr Ni O P Si grnJdm.

I Not added.

Analysis reflected alloy 5 contained 0.076% A1 and 0.008% B.

The chromium and nickel contents are nominal percentages.

Norm-A11 alloys contained less than 0.6 manganese except 21-4-N which contained about 9.29% manganese (and also 0.43% N).

included in Table 1 above are Alloys A through F, and also a commercial specimen of one of the most conventionally used exhaust valve steels to wit, 2 lCr-4Ni-9Mn-N (herein 21-4-N). Alloy A, an alloy outside the invention, as are all other alloys hereafter identified by a letter, reflects the excellent corrosion resistance attainable with stainless steels of relatively high purity and which contain percentages of chromium and nickel in accordance herewith but to which neither carbon nor phosphorus was added. While the corrosion resistance of such alloys in terms of the lead oxide crucible test are excellent, being one-half that of the commercially used 21-4-N alloy, in terms of hardness and strength such steels are, however, completely inadequate. Alloy B does illustrate the adverse effect to be expected with an alloy similar to Alloy A but which contains a fairly high amount of silicon, an effect reversed by the beneficial effect of carbon, Alloy C. The subversive influence of phosphorus on corrosion is depicted by Alloy D (compare with Alloy C). Both Alloys C and D suffer by inferior high temperature stress rupture strength.

In contrast with Alloys A-D, Alloys 1 through 5, alloys within the invention, all exhibited good corrosion resistant qualities as contemplated herein. 1t is interesting to compare Alloy 1 with Alloy E. These alloys are reasonably close in composition except that in the latter the ratio of carbon to phosphorus is not more than 1.25:1 and far removed from a ratio of at least 1.521. This comparison does serve as an indication of the benefits that flow from correlating the amounts of carbon and phosphorus as required herein.

In Table 11 are reported the results concerning the behavior of various alloys (including a specimen of 21-4-N) at elevated temperatures. In this connection hot hardness was determined at l,400 F. and stress rupture properties were ascertained at 1,350 F. and/or 1,500 F., the stresses being 35,000 and 10,000 p.s.i., respectively. Also included are representative toughness data as determined by the conventional Charpy V- Notch test at 70 F. Each of the alloys was in the wrought condition and solution treated at 2,300 F. for 1 hour, water quenched and aged 4 hours at l,300 F.

austenitic stainless for the head. In view of this and to restrict the possible occurrence of overaging and also to minimize liquation of low melting constituents such as carbon and phosphorus, a rapid welding technique was self-suggestive and flash butt welding was decided upon.

Using this process, Alloys 1 and 5 were flash butt welded to commercially produced AISI 1045 steel. The specimens were in the form of i-inch hot-rolled bar, Alloys 1 and 5 having TABLE 11 Stress Percent Hardness rupture 1,350 1,500 CVN,

Alloy Cr Ni C P Si Fe F.,Ra R.I., Rc hours hours ft.-lbs.

20. 7 30. 4 0.32 0. I 0. 05 Bal. 59 33.0 93. 7 17. 5

27. l 30. 9 0. 37 0. 014 0. 045 Bal. 22. 0 4. 9 26.5

27. 0 29. 9 0. 25 0.10 0.035 Bal. 28.0 41. 1 20. 5

20. 5 3. 7 0.56 0.02 0. 14 Bal. 45

*Contained 0.12% Al.

"Discontinued after 500 hours. R.T.= Room Temperature.

Little comment is necessary regarding the above data other than to possible mention that in both Alloys G and H either the carbon plus phosphorous or phosphorous was insufficient. As referred to herein, hot workability is extremely important. In Table III there are set forth data illustrating generally the adverse effects that might be expected with the use of high hot working temperatures during initial ingot breakdown. The specimens were prepared as -pound air induction melts and were deoxidized with aluminum and calcium-silicon. Twentyfive-pound z-inch square ingots and five pound cylindrical bars were cast. The purpose of the bars, which were sectioned into lr-inch slugs, was to determine (using hot upsetting) the range of forging temperatures. Grinding of the ingots was found important in removing fine surface crazings and hot tears. This step was adapted as standard procedure for ingot conditioning. The ingots were forged and rolled over a temperature range of 1,800 to l,950 F. ingots which were successfully reduced to two inch square billets were reheated and rolled to onehalf inch bars. The carbon and phosphorus contents are set forth in Table III, the steels otherwise nominally containing 27 percent chromium and 30 percent nickel.

TABLE 111 Percent Hot working Alloy C P treatment Comments 6 0. 26 0.27 Conditioned, forged No cracking, bar

at 1,950 F. to 2" quality acceptasq., rolled at ble. 1,950 F. to 56 bar.

4 0.32 0. 16 Conditioned, forged to N0 cracking.

3" sq. at 1,950 F., rolled on plate mill to 2 sq., rolled to bar.

8 0. 4O 0. 16 Conditioned, forged Slight cracking in to 3 sq. at, forging, bar 1,950 F., rolled to quality fair.

2" sq. at 1,950 F.,

on late mill, rolled to bar. I 0. 29 0. 26 Forged 2,250 F Severe hot tearing. K 0.44 0.26 do Do.

7' While some degree of cracking might be expected on an experimental scale (Alloy 8), the severe hot tearing experienced with the high hot working temperature of 2,250 F. for Alloys J and K is a good indication that a temperature of about 2,150 F. should not be exceeded during the initial stages of hot working.

Finally, as to weldability, steels within the invention were flash butt welded. In this connection, it might be mentioned mirfcbmi'nercial practice duplex type valves are ordinarily welded subsequent to heat treatment. This, therefore, requires, of course, the minimum of degradation of properties with respect to both types of materials used in the valve, usually a carbon or low alloy steel for the stem and an been solution treated one hour at 2,300 F., water quenched and aged 4 hours at 1,300 F., the A181 1045 steel specimens having been austenitized one hour at l,500 F., water quenched and tempered one hour at 700 F The welding rod specimens were machined to one-quarter inch and then butt welded using commercial equipment. Dye penetrant, X-ray radiography and metallographic (500X) inspection tests were conducted. None of the tests revealed the presence of defects. Moreover, each of the weldments exhibited considerable bend ductility. Thus, it was concluded that steels contemplated in accordance herewith manifest good weldability characteristics.

While the invention as above set forth is primarily directed to hot workable duplex valve steels, the single-piece variety is not excluded. Of course, it is contemplated that the steels may be used wherever the combination of corrosion resistance and mechanical characteristics as described herein would be useful.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An austenitic stainless steel containing from 18 to 30 percent chromium, from 20 to 40 percent nickel, from 0.25 to less than about 0.95 percent carbon, from about 0.1 to about 0.35 percent phosphorus with the proviso that the carbon and phosphorus are correlated as to represent a point within the area ABDEFA of the accompanying drawing, up to 0.5 percent silicon, up to about 2 percent manganese and the balance essentially iron.

2. A steel in accordance with claim 1 in which the chromium content is from 25 to 29 percent.

3. A steel in accordance with claim 1 containing from about 25 to about 33 percent nickel.

4. A steel in accordance with claim 1 in which the chromium and nickel are correlated such that the sum of 2.8 X %Cr plus %Ni is at least about 92.

5. A steel in accordance with claim 1 in which the ratio of carbon to phosphorus is at least 1.5: 1.

6. A steel in accordance with claim 1 in which the percentage of carbon plus phosphorus does not exceed 0.75 percent.

7. A steel in accordance with claim 1 in which the phosphorus content does not exceed 0.3 percent.

8. A steel in accordance with claim 1 in which the sum of the phosphorus and any silicon does not exceed about 0.65 percent.

9. A steel in accordance with claim 1 in which the percentage of carbon and phosphorus is correlated so as to represent centage of carbon plus phosphorus does not exceed about 0.6 1

percent.

12. A steel in accordance with claim 10 in which the percentage of phosphorus plus carbon does not exceed about 0.5 percent and any silicon does not exceed about 0.1 percent 13. A steel in accordance with claim 1 which has been solution treated at a temperature of from l,800 to about 2,350 F. and aged at a temperature of from 1, 1 00 F. to l,400 F.

14. A steel in accordance with claim 13 in which the solution treating temperature is at least about 2,200 F.

15. A steel in accordance with claim 10 which has been solution treated at a temperature of from about 2,200 to 0 about 2,350 F. and aged at about l,250 to 1,350 F. 

2. A steel in accordance with claim 1 in which the chromium content is from 25 to 29 percent.
 3. A steel in accordance with claim 1 containing from about 25 to about 33 percent nickel.
 4. A steel in accordance with claim 1 in which the chromium and nickel are correlated such that the sum of 2.8 X %Cr plus %Ni is at least about
 92. 5. A steel in accordance with claim 1 in which the ratio of carbon to phosphorus is at least 1.5:1.
 6. A steel in accordance with claim 1 in which the percentage of carbon plus phosphorus does not exceed 0.75 percent.
 7. A steel in accordance with claim 1 in which the phosphorus content does not exceed 0.3 percent.
 8. A steel in accordance with claim 1 in which the sum of the phosphorus and any silicon does not exceed about 0.65 percent.
 9. A steel in accordance with claim 1 in which the percentage of carbon and phosphorus is correlated so as to represent a point within the area ABCGHFA of the accompanying drawing.
 10. A steel in accordance with cLaim 1 containing from 25 to 29 percent chromium, from 25 to 33 percent nickel, from 0.3 to 0.5 percent carbon, from 0.12 to 0.25 percent phosphorus, up to about 0.3 percent silicon, and the balance essentially iron.
 11. A steel in accordance with claim 10 in which the percentage of carbon plus phosphorus does not exceed about 0.6 percent.
 12. A steel in accordance with claim 10 in which the percentage of phosphorus plus carbon does not exceed about 0.5 percent and any silicon does not exceed about 0.1 percent.
 13. A steel in accordance with claim 1 which has been solution treated at a temperature of from 1,800* to about 2,350* F. and aged at a temperature of from 1,100* F. to 1,400* F.
 14. A steel in accordance with claim 13 in which the solution treating temperature is at least about 2,200* F.
 15. A steel in accordance with claim 10 which has been solution treated at a temperature of from about 2,200* to about 2,350* F. and aged at about 1,250* to 1,350* F. 